Silicon oxide slurry and method for producing such slurry

ABSTRACT

The present invention relates to a slurry containing water, amorphous silica panicles having a particle size less than 1 micron and silica flour with a particle size between 2-200 microns. In order to stablize the slurry, the slurry contains a polysaccharide. The invention further relates to a method for the production of a slurry containing water, amorphous silica having a particle size below 1 micron, and silica flour with a particle size between 2-200 microns, where a polysaccharide is added to a slurry of water and amorphous silica, whereafter the silica flour is mixed into the slurry of amorphous silica.

This application is a 371 of PCT/NO03/00135, filed 24 Apr 2003.

FIELD OF INVENTION

The present invention relates to an additive for oil well cementcontaining amorphous silica particles and silica flour and to a methodfor producing such additive

BACKGROUND ART

From EP-B 467921 it is known a method for the mixture of silicon dioxideto a hydraulic cement slurry, where amorphous silica with particlessmaller then 1 μm is mixed with water to form an aqueous slurry ofmicrosilica whereafter silica flour with particles with a size from2-200 μm are mixed into the suspension of amorphous silica particles.Finally, the produced liquid slurry is added to a cement slurry in amixing tank.

The cement slurry produced according to the method of EP-B 467921 isparticularly useful for cementing oil wells at high temperatures ofabove 100° C. as it has been found that this prevent long time reductionof strength of the cement.

In order to make full economic and practical use of the method accordingto EP-B 467921 the slurry containing amorphous silica and silica flourshould be produced and transported to the cementing site for mixing tothe cement slurry shortly before cementing. For use in cementing of oilwells offshore and on shore, this means that the slurry of microsilicaand silica flour must be produced ashore and transported to a rig beforeit can be mixed into a cement slurry. Unfortunately it has been foundthat the suspension of amorphous silica and silica flour shows a strongtendency of settling resulting in such a short shelf life that thesuspension cannot be transported for mixing into cement slurries. Thereis thus a need for a slurry of the type disclosed in EP-B 467921 whichhave a reduced tendency of settling and which is compatible with cementslurries.

DISCLOSURE OF INVENTION

By the present invention it has now been provided a slurry of amorphoussilica and silica flour and a method for production of such slurry whichshows a strongly reduced tendency of settling and which is compatiblewith cement slurries and which does not contain any toxic additives.

Thus, according to a first aspect, the present invention relates to aslurry containing water, amorphous silica particles having a particlesize less then 1 micrometer and silica flour having a particle sizebetween 2 and 200 μm, which slurry is characterized in that it containsa polysaccharide.

According to a preferred embodiment the polysaccharide is a cellulosederivate selected among xanthan, carboxymethylcellulose,hydroxymethylcellulose, hydroxyethylcellulose or mixtures of thesecompounds.

According to a further preferred embodiment the slurry contains between0.01 and 3 grams of polysaccharide pr. litre of slurry and morepreferably between 0.05 and 1.5 grams of polysaccharide pr. litre ofslurry.

The amount of polysaccharide pr. litre of slurry is adjusted accordingto the chain length of the polysaccharide. When using polysaccharideshaving a short chain length, the amount of polysaccharide in the slurryis in the upper end of the ranges and when using polysaccharide having along chain length the amount is in the lower part of the range.

In order to further increase the stabilizing effect of thepolysaccharide, the slurry optionally contains one or more of dextrin,guar gum and locust bean gum.

The slurry according to the invention may contain varying amounts ofamorphous silica and silica flour, but the amount of amorphous silica isgenerally between 15-50% by weight based on the weight of the slurry andthe amount of silica flour is generally between 5 and 60% by weightbased on the weight of the slurry.

The total amount of dry matter in the slurry is preferably between 40and 80% by weight based on the weight of the slurry.

It has surprisingly been found that the slurry according to theinvention is very stable and shows little or no tendency of settlingeven after two to three months storage. The slurry can thus be storedand transported to the sites where it is mixed into cement slurries.Further it has been found that the slurry according to the invention iscompatible with cement slurries in that it gives an acceptable rheologyof the cement slurries.

According to a second embodiment the present invention relates to amethod for the production of a slurry containing water, amorphous silicahaving a particle size below 1 μm, and silica flour having a particlesize between 2 and 200 μm, which method is characterized in that apolysaccharide is added to a slurry of water and amorphous silica,whereafter the silica flour is mixed into the slurry of amorphoussilica.

According to a preferred embodiment the polysaccharide is preconditionedin a water-containing medium for at least 15 minutes before it is addedto the slurry of water and amorphous silica.

The polysaccharide is preferably preconditioned in a slurry of water andamorphous silica.

According to a preferred embodiment the polysaccharide added to theslurry of amorphous silica and water is a cellulose derivate suchxanthan, carboxymethylcellulose, hydroxymethylcellulose,hydroxyethylcellulose or mixtures of these compounds.

Preferably the silica flour is mixed into the slurry of water andamorphous silica using a high shear energy mixer.

The polysaccharide is preferably added to the slurry of amorphous silicaand water in an amount necessary to provide a content of polysaccharidein the final slurry of 0.01 to 3 grams pr. litre and more preferably inan amount of 0.05 and 1.5 grams pr litre.

It has surprisingly been found that the addition polysaccharide to theslurry of amorphous silica and silica flour results in a stable slurrywith a strongly reduced tendency of settling resulting in a stronglyincreased shelf life of the slurries. The preconditioning of thepolysaccharide has shown to even further improve the stability of thefinal slurry. Further it has been found that the addition ofpolysaccharides gives acceptable rheological properties for oil wellcement slurries containing the slurries according to the invention.Finally polysaccharides are generally non-toxic compounds which areapproved to be used in connection with oil well cements.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1

0.44 grams of xanthan pr. litre of final slurry was added to a slurry ofamorphous silica and water containing 50% by weight of amorphous silica.The xanthan had been preconditioned in a small part of the slurry ofwater and amorphous silica for 24 hours before it was added. 850 gramspr litre of final slurry of silica flour having a mean particle size of25 μm was thereafter added to the slurry using a high shear mixer. Thefinal slurry was stored in 100 ml glass cylinders for 34 days. Thesamples in the glass cylinders had a 2 mm top layer of water after 34days, but no resistance was found when lowering a rod to the bottom ofthe cylinder. The samples were very fluid and when the cylinders wereemptied there was not found and settling in the bottom of the cylinders.

For comparison purposes an identical slurry was made, but without theaddition of xanthan. After 7 days it was found that excessive settlinghad occurred. The cylinder had a 25 mm top layer of water and a hardlayer was found in the bottom of cylinder which layer could not beredispersed. The hard layer consisted of silica flour.

EXAMPLE 2

A slurry according to the invention was made in the same way asdescribed in Example 1, except that 0.22 grams of hydroxyethylcellulosewas added instead of xanthan. The slurry was filled into a glasscylinder. After 1 week storage the slurry had a 10 mm top layer of waterand no hard bottom layer was found.

EXAMPLE 3

A slurry according to the invention was made in the same way asdescribed in Example 1., except that 0.22 grams carboxymethylcellulosewas added instead of xanthan. The slurry was filled into a glasscylinder. After 1 week storage the slurry had a 8 mm top layer of waterand no hard bottom layer was found.

The above examples show that the slurry according to the invention has astrongly reduced settling compared to the prior art slurry.

EXAMPLE 4

A slurry of amorphous silica and silica flour containing xanthanaccording to the present invention and made according to Example 1 wasadded to an oil well cement slurry in an amount necessary to provide atotal SiO2 content in the cement slurry of 35% by weight based on theweight of cement. The slurry had a density of 1.9 g/cm³. This slurry isdenoted slurry A

For comparison purpose it was made an identical cement slurry B exceptthat cement slurry B was made by adding a slurry of amorphous silica andsilica flour that did not contain a polysaccharide additive.

The rheological properties of the two cement slurries were measuredaccording to API Specification 10 and the results are shown in Table 1.

TABLE 1 Cement slurry A B Rheology (20° C.) Plastic viscosity, cp 64.548 Yield point, lb/100 ft³ 8.5 3 Rheology (88° C.) Plastic viscosity, cp45 37.5 Yield point, lb/100 ft³ 6 1.5

As can be seen from Table 1, the rheological properties of a cementslurry containing the slurry of amorphous silica and silica flouraccording to the invention does not deviate much from the sameproperties of the prior art slurry B and are well within the rangesaccepted for oil well cement slurries.

1. A stable, storable slurry consisting essentially of water, amorphoussilica particles having a particle size less than 1 um, silica flourwith a particle size between 2-200 um, and a preconditionedpolysaccharide as a stabilizer, wherein the polysaccharide has been in awater medium for at least 15 minutes before it is added.
 2. The slurryaccording to claim 1, wherein the polysaccharide is selected from thegroup consisting of xanthan, carboxymethylcellulose,hydroxymethylcellulose, hydroxyethylcellulose and mixtures of thesecompounds.
 3. The slurry according to claim 1, wherein the slurrycomprises between 0.01 and 3 grams of polysaccharide per liter ofslurry.
 4. The slurry according to claim 1, wherein the slurry comprisesbetween 0.05 and 1.5 grams of polysaccharide per liter of slurry.
 5. Theslurry according to claim 1, wherein the slurry further comprises one ormore of dextrin, guar gum and locust bean gum.
 6. A method for theproduction of a slurry containing water, amorphous silica having aparticle size below 1 um, and silica flour with a particle size between2-200 um, comprising: adding a polysaceharide to a slurry of water andamorphous silica, where after mixing the silica flour into the slurry ofamorphous silica.
 7. The method according to claim 6, wherein thepolysacoharide is in a water containing medium for at least 15 minutesbefore adding to the slurry.
 8. The method according to claim 7, whereinthe water medium is a slurry of water and amorphous silica.
 9. Themethod according to claim 6, wherein the silica flour is mixed into theslurry of water and amorphous silica using a high shear energy mixer.10. The method according to claim 6, wherein the polysaccharide is addedto the slurry of amorphous silica and water in an amount necessary toprovide a content of polysaccharide in the final slurry of 0.01 to 3grams per liter.
 11. The method according to claim 10, wherein thepolysaccharide is added to the slurry of amorphous silica and water inan amount necessary to provide a content of polysaccharide in the finalslurry of 0.1 and 1.5 grams per liter.
 12. The method according to claim6, wherein the polysaccharide is selected from the group consisting ofxanthan, carboxymethylcellulose, hydroxymethylcellulose,hydroxyethylcellulose and mixtures of these compounds.
 13. The methodaccording to claim 6, wherein the product further comprises one or moreof dextran, guar gum and locust bear gum.